1. Field of the Invention
This invention relates to a toner used in electrophotography, electrostatic recording, electrostatic printing and toner jet recording (magnetic recording).
2. Related Background Art
A number of methods are known as methods for electrophotography (see, e.g., U.S. Pat. No. 2,297,691). In general, copies are obtained by forming an electrostatic latent image on a photosensitive member by various means utilizing a photoconductive material, subsequently developing the latent image by the use of a toner to form a visible image, and transferring the toner (toner image) to a recording material (transfer material) such as paper as occasion calls, followed by fixing by the action of heat and/or pressure. The toner that has not transferred to and has remained on the photosensitive member is cleaned by various means, and then the above process is repeated.
In recent years, it has been put forward to improve such copying apparatus toward higher image quality, smaller size, lighter weight, higher speed and higher reliability with a high demand from users, where the performance of products have severely been investigated. Also, the such image-forming apparatus not only have been used as copying machines for office working to take copies of originals, but also have long been used as digital printers for outputting data from computers or used for copying highly minute images such as graphic designs. In more recent years, with tremendous spread of digital cameras, there is an increasing demand for high-color printers for outputting photographs taken therewith. In the meantime, it has become more and more necessary to consider how to deal with environmental problems, how to deal with energy saving, and so forth.
The step of development may be given as the step of forming electrophotographic images that is difficult for the achievement of higher image quality, higher minuteness and higher stability as those demanded by users.
In electrophotography, the step of developing an electrostatic latent image is the step of utilizing electrostatic mutual action between toner particles having been charged and the electrostatic latent image to form a visible image on the electrostatic latent image. Developers with which electrostatic latent image are developed by the use of toners include a magnetic one-component developer making use of a toner formed of a resin and a magnetic material dispersed therein, a non-magnetic one-component developer which performs development by charging a non-magnetic toner electrostatically by means of a charge-providing member such as an elastic blade, and a two-component developer formed of a blend of a non-magnetic toner with a magnetic carrier.
At present where the technique to expose the photosensitive member to light using small-diameter laser beams or the like has advanced and electrostatic latent images have come minute, it has been put forward to make both toner particles and carrier particles have smaller diameters in any of the above developing systems so that faithful development can be performed on the electrostatic latent images and images can be reproduced in a higher image quality. In particular, it is often attempted to make toners have a smaller average particle diameter to improve image quality.
Making toners have a smaller average particle diameter is an effective means for improving image characteristics, in particular, graininess and character reproducibility. However, it has problems to be solved, in respect of specific image quality items, in particular, fog at the time of extensive printing, melt adhesion to photosensitive member, toner scatter and so forth.
Such problems are firstly caused by a lowering of charge quantity of toners that results from the two things that i) the use of toners over a long period of time causes deterioration of external additives having been added to toner particles and ii) charge-providing members such as a developing sleeve and a carrier and a toner layer thickness control member for keeping the coating of toner on the sleeve to a stated quantity are contaminated by the toner and the external additives, i.e., toner-spent comes about. These phenomena tend to occur as a result of making toners have smaller particle diameters. To amplify the situation, triboelectric charging is performed by means of physical external force such as contact and collision between the toner and the sleeve in the case of one-component developers and between the toner and the carrier in the case of two-component developers, and hence all the toner, the charge-providing members (sleeve and carrier) and the toner layer thickness control member may necessarily be damaged. For example, in the toner, the external additives added to its toner particle surfaces may come buried in toner particles or toner components may come off. In the charge-providing members and the toner layer thickness control member, they may be contaminated with toner components including the external additives, or coat components with which the charge-providing members are coated in order to stabilize charge properly may wear or be broken. Because of such damage, the initial characteristics of the developers become not maintainable with an increase in the number of copying times to cause fog, in-machine contamination and variations of image density. This phenomenon becomes conspicuous especially as the image-element units of electrostatic latent images are made minuter.
Secondly, the above problems may arise because, where an original having a high image area percentage is used and where the toner is fed onto the charge-providing members in a large quantity, it takes a time until the toner having been fed is uniformly charged and the toner uncharged participates in development. This phenomenon occurs remarkably especially when the toner has small diameter and has low fluidity. Any image defects thereby caused tend to come into question when multi-superimposed images are formed in full-color image formation, and are especially required to be remedied. As a countermeasure for this problem, it has been main to make studies on triboelectric series and resistance of the charge-providing members. As the toner, it is also studied to improve various charge control agents so that the toner can quickly be charged.
As the magnetic carrier used in the two-component developer, an iron powder carrier, a ferrite carrier or a carrier coated with a resin obtained by dispersing fine magnetic-material particles in a binder resin is known in the art. In particular, a developer making use of a resin-coated carrier obtained by coating carrier core material surfaces with a resin is preferably used because it can have proper electrical resistance, has superior charge controllability and can relatively easily be improved in environmental stability and stability with time.
In order to overcome an insufficiecny in charging to the small-particle-diameter toner as stated above, it is also a preferable means especially in the two-component developer to make the carrier have a small particle diameter. This, however, tends to make toner-spent resistance poor as the carrier has a larger specific surface area.
To solve these problems, it is attempted to use the carrier in a large quantity. This, however, goes against the downsizing of copying machine or printer main bodies, and is not practical.
Meanwhile, steps which are most important for satisfying the demand of users and are technically difficult include the fixing step.
With regard to the fixing step, various methods and assemblies have been provided. The most commonly available method at present is a pressure-and-heating system making use of a heated roller, film or belt.
The pressure-and-heating system is a system in which the toner image surface of a sheet to which toner images are to be fixed (hereinafter “fixing-medium sheet”) is made to pass the surface of a fixing member having a heating source, which member has a surface formed of a material with releasability to the toner (such as silicone rubber or fluorine resin), in contact with a pressure member under application of its pressure against the fixing member to perform fixing. This system is very effective in high-speed electrophotographic copying machines because the toner image on the fixing-medium sheet comes into contact with the surface of the fixing member as a heating member under application of pressure and hence the thermal efficiency in fusing the toner image onto the fixing-medium sheet is so good that the toner image can rapidly be fixed. In this system, however, since the toner image comes into pressure contact with the heating member in a molten state, part of the toner image may adhere, and be transferred, to the heating member surface to contaminate the next fixing-medium sheet (what is called “offset phenomenon”). Accordingly, it is regarded as one of essential conditions to make the toner not adhere to the heating member.
For this reason, for the purpose of preventing the offset, a method in which an oil such as silicone oil is fed to the fixing member to apply the oil uniformly on the fixing member is also used in color copying machines.
This method is very effective in preventing the offset of the toner. However, it requires a unit for feeding such an offset-preventive fluid, and has a problem that it makes the fixing assembly complicate, providing an inhibitory factor in the designing of compact and inexpensive systems. Further, in the case of an overhead projector transparency film or sheet (OHT film or sheet) needed increasingly as its use for presentation, it has a low oil absorption capacity as being different from paper, and hence the stickiness of the OHT film surface has come into question. In the case of paper as well, it has a problem that its surface is not inscribable with a pen using water-based ink or the like because of the oil absorbed therein. Under such background, it is strongly sought to provide full-color toners that are fixable in an oilless system or a system in which the oil is applied in a small quantity.
Under such circumstances, oilless fixing or small-quantity oil application fixing has been materialized in color toners as well, by incorporating a release agent into toner particles.
It is known to incorporating the release agent into toner particles (see, e.g., Japanese Patent Publication No. S52-3304 and Japanese Patent Application Laid-Open No. S57-52574).
Incorporation of the release agent into toner particles is also disclosed in a large number (see, e.g., Japanese Patent Applications Laid-Open No. H03-50559 and No. H02-79860).
The release agent is used in order to improve anti-offset properties at the time of high-temperature fixing or low-temperature fixing of toners, or to improve fixing performance at the time of low-temperature fixing. On the other hand, it may lower anti-blocking properties of toners, may lower developing performance of toners because of in-machine temperature rise, or may lower developing performance of toners because of exudation of the release agent to toner particle surfaces when the toners are left over a long period of time.
It is also disclosed that specifying the modulus of elasticity of toner particles containing a release agent makes it possible to perform oilless fixing. In publications, it is certainly disclosed that specifying viscoelasticity in the vicinity of fixing preset temperatures 150° C. and 170° C. enables achievement of both OHT film transparency and high-temperature anti-offset properties (see Japanese Patent Applications Laid-Open No. H06-59502 and H08-54750). However, in the case of high-speed fixing, in which the temperature of the heating member drops violently at the time of continuous paper feed, the method disclosed has some problems in respect of things relating to fixing, such as faulty fixing at the time of low-temperature fixing, what is called a low-temperature offset phenomenon and faulty paper delivery and placement, and in respect of how to ensure stable developing performance over a long period of time.
Some description is added in regard to the above faulty paper delivery and placement. As a problem in the case of the oilless fixing or small-quantity oil application fixing, the transfer sheet is put out in such a form that it is pulled toward the fixing member after its leading end on the paper delivery side has passed the fixing nip. This is a phenomenon which occurs because of a shortage of releasability between the toner melt surface and the fixing member. In this case, the problem of faulty placement may arise on the paper delivered in a large number of sheets. Also, where the above phenomenon occurs at a serious level, the transfer sheet may wind around the fixing member to cause the faulty paper delivery. In order to prevent this faulty paper delivery, it is attempted to keep a member such as a separation claw in contact with the fixing member or to provide the former in non-contact and bring it into touch with the latter. In the case of keeping the former in contact, the offset toner having stagnated at the separation claw or the like may enlarge the contact pressure on the fixing member to scratch the fixing member surface, so that the fixing performance at that part may lower to cause a difference in gloss from the other part, making the quality level of fixed images different only at that part. In addition, the toner having stagnated at the separation claw may come off at certain timing and transfer to the pressure member to cause what is called back staining where the back of the image-fixed transfer sheet stains. In order to lessen such a phenomenon, it is attempted to bring into touch therewith a web or the like impregnated with silicone oil or the like. This, however, goes against the downsizing of copying machine or printer main bodies as stated above. The phenomenon of wind-around may more occur as the affinity of the toner for the fixing member is higher, and tends to occur more seriously as the fixing speed is higher and the fixing temperature is lower as the makeup of fixing.
As a further demand in the fixing step, toners may be given which are fixable at a low temperature correspondingly to the achievement of energy saving and high speed in copying machine or printer main bodies. In particular, in the formation of full-color images, colors are reproduced using three color toners of coloring matter's three primary colors, yellow, magenta and cyan colors, or four color toners consisting of these color toners and a black toner added thereto. Accordingly, in fixing multi-color toner images onto paper and in fixing them onto the overhead projector transparency sheet (OHT), color reproducibility and transmission properties must be satisfied. Thus, their formation involves a high degree of technical difficulty.
In order to solve these problems, it is preferable to use a resin having sharp-melt properties. In particular, it is attempted to incorporate a polyester resin into toner particles.
The polyester resin affords superior low-temperature fixing performance, but, on the other hand, because of the acid value and hydroxyl value it has, makes it difficult to control charge quantity when made into a toner. Stated specifically, it may make the toner greatly dependent on environment, such that the toner may be charged in excess (what is called charge-up) in an environment of low humidity and charged insufficiently in an environment of high humidity, and it may make the toner have a low rise speed of charging.
As a polymerization catalyst used for producing such a polyester resin for toners, it has commonly been attempted to use a tin type catalyst such as dibutyltin oxide or an antimony type catalyst such as antimony trioxide. These techniques have some problem in respect of fixing performances such as low-temperature fixing performance and high-temperature anti-offset properties which are demanded in full-color copying machines in recent years, how to satisfy color reproducibility such as color mixing properties and transparency, rise characteristics of charging, and how to stably control charge quantity of toners.
Accordingly, it is proposed to use a titanate of a diol as the polymerization catalyst (see Japanese Patent Application Laid-Open No. 2002-148867). It is also proposed to use a solid titanium compound as the polymerization catalyst (see Japanese Patent Application Laid-Open No. 2001-64378). Although the use of a titanium compound as the polymerization catalyst restrains the phenomenon of charge-up of toners, these proposals have not made the rise characteristics of charging well satisfactory.
The use of the resin having sharp-melt properties also usually tends to cause a problem on high-temperature anti-offset properties when the toner melts in the step of heat-and-pressure fixing, because the binder resin has a low self-cohesive force. Accordingly, a relatively highly crystalline wax as typified by polyethylene wax and polypropylene wax is used as the release agent in order to improve the high-temperature anti-offset properties at the time of fixing.
However, in the toners for full-color images, when images are projected using an overhead projector (OHP), their transparency may be obstructed and the projected images may have a low chroma or brightness, because of a high crystallizability of the release agent itself or a difference in refractive index between the release agent and the OHT sheet.
Accordingly, to solve these problems, a method is proposed in which a wax having a low crystallinity is used (see Japanese Patent Applications Laid-Open No. H04-301853 and No. H05-61238). As waxes having a relatively good transparency and a low melting point, montan type waxes are available. Use of such montan type waxes is proposed in a large number (see Japanese Patent Applications Laid-Open No. H01-185660, No. H01-185661, No. H01-185662, No. H01-185663 and No. H01-238672). These waxes, however, have some problems for well satisfying all the transparency in OHP and the low-temperature fixing performance and high-temperature anti-offset properties at the time of heat-and-pressure fixing.
In addition, in any of the above toners incorporated with the release agent, those which afford good developing performance, in particular, the rise characteristics of charging stably over a long period of time do not exist because of the presence of the release agent on toner particle surfaces.
As discussed above, under the existing conditions, any toner has not yet been made available which has achieved both the fixing performance that can realize low-cost, compact and high-speed machines and the developing performance that can satisfy image quality level over a long period of time.